1. Field of the Invention
The present invention relates to a method of recording an identifier (i.e., an ID or an ID mark) on a plate member and a set of photomasks for use in such a method. Preferred embodiments of the present invention are effectively applicable for use to inscribe an identifier not only on a ceramic wafer for a magnetic head, for example, but also on a semiconductor wafer or any other plate-type member.
2. Description of the Related Art
Recently, a thin-film magnetic head having any of various structures often includes a magnetic head slider for use in a hard disk drive (HDD), a tape storage and a flexible (or floppy) disk drive (FDD), for example. Examples of wafers for such a thin-film magnetic head include sintered wafers having compositions such as Al2O3—TiC, SiC and ZrO2.
FIG. 1A illustrates a typical thin-film magnetic head slider 10. On its tracking side, this magnetic head slider 10 includes two side rails 11 that will be opposed to the surface of a magnetic disk. The surface of the thin-film magnetic head slider 10 on which the side rails 11 are provided is sometimes called an “air bearing surface (ABS)”. If the magnetic disk is rotated at a high velocity by a motor, for example, while the surface of the magnetic disk is pressed lightly by the side rails 11 of the magnetic head slider 10 by way of a head suspension, then an air layer will be formed on the surface of the magnetic disk and will reach the back surface of the air bearing surface of the slider 10. As a result, the magnetic head slider 10 is slightly lifted up. In this manner, the magnetic head slider 10 can perform read and write operations on the magnetic disk while “flying” near the surface of the disk so to speak.
A thin film 12, which causes a magnetic interaction with a storage medium such as the magnetic disk, is deposited on one end surface of the magnetic head slider 10. The thin film 12 is used to form part of an electrical/magnetic transducer. To indicate the type of the product, an identifier (ID or ID mark) 13 such as a serial number is inscribed on the other end surface of the magnetic head slider 10. Methods of inscribing an identifier 13 on sintered wafers are disclosed in Japanese Laid-Open Publications Nos. 9-81922, 10-134317 and 11-126311, for example.
In a typical manufacturing process, the magnetic head slider 10 is obtained by cutting out a bar 20 shown in FIG. 1B from a sintered 1 shown in FIG. 1C and then dicing the bar 20 into a great number of chips. In FIG. 1C, the end surface 4 of the wafer 1 is parallel to the air bearing surface of the magnetic head slider 10 shown in FIG. 1A.
Recently, as the sizes of such a thin-film magnetic head have been decreased to reduce the sizes and weight of an electronic appliance, the thickness of the wafer 1 (corresponding to the length L of the magnetic head slider 10) and the thickness T of each bar 20 (corresponding to the height of the magnetic head slider 10) have also been reduced. For example, a magnetic head slider, which is called a “pico-slider”, has a length L of about 1.2 mm and a thickness T of about 0.3 mm. As for a magnetic head slider of such drastically reduced sizes, the sizes of characters to be inscribed on the slider should also be reduced correspondingly.
In the prior art, a laser marking method is often used to inscribe the identifier 13. In the laser marking method, the identifiers 13 shown in FIGS. 1A and 1B are written on the back surface 3 of the wafer 1 that is yet to be divided into the bars 20. After the ID marking printing process step is finished, various thin films 12 are stacked on the surface 2 of the wafer 1.
Hereinafter, the conventional laser marking method will be described briefly with reference to FIG. 2.
In the laser marking method, the back surface 3 of the wafer 1 is locally irradiated with a laser beam 6 that has been condensed by a lens 5, thereby rapidly heating and vaporizing the irradiated portion of the wafer 1. In this case, a tiny concave portion is formed on the back surface 3 of the wafer 1, while the material of the sintered wafer 1 is scattered around and just a portion of the scattered material is deposited on the wafer 1 again. By scanning the back surface 3 of the wafer 1 with the laser beam 6, the concave portions can be arranged so as to form an arbitrary pattern on the back surface 3 (which will be herein referred to as a “concave pattern”). Any of various types of identifiers 13 can be written at an arbitrary location on the wafer 1 by forming a concave pattern, which is made up of alphanumeric and/or numeric characters or a barcode, on the back surface 3 of the wafer 1.
A laser marking method as described above, however, has the following drawbacks.
Firstly, the portion of the sintered material that has been scattered around as a result of the exposure to the laser beam is likely adsorbed or deposited as dust onto the inscribed characters, thus causing a contamination problem in many cases.
Secondly, the edges of the inscribed characters are often burred through the exposure to the laser beam. Thus, a deburring processing step needs to be carried out.
A photolithographic method was used as an alternative identifier marking method that can avoid these problems. In the photolithographic process, first, the back surface 3 of the wafer 1 is coated with a photoresist layer. Next, the photoresist layer is exposed to a radiation through a particular photomask, thereby forming a latent image of the identifier in a desired region of the photoresist layer. Thereafter, the exposed photoresist layer is developed so as to transfer a pattern representing the identifier onto the photoresist layer. Finally, the wafer is selectively etched away by using the patterned photoresist layer as an etching mask. In this manner, the identifier can be written on the wafer. According to this photolithographic process, a fine-line pattern can be formed and any of various types of identifiers can be clearly recorded even within a narrow region.
The conventional photolithographic process described above is far from being cost effective for the following reasons. Specifically, when different types of identifiers should be written on multiple wafers, this photolithographic process requires the same number of photomasks as that of the wafers. However, photomasks are normally expensive. For example, just one photomask sometimes costs hundreds of thousand yen (i.e., over $1,000). For that reason, as the number of photomasks needed increases, the overall processing cost of this photolithographic process increases by leaps and bounds, thus constituting a great obstacle to desired cost reduction.